[dev.typeparams] go/types: use Checker-provided type parameter IDs when possible

Incrementing type parameter subscripts for each type checking pass is
distracting for an interactive program where packages are type-checked
on each keystroke.

We should perhaps hide the type parameter ID altogether, but for now at
least add a layer of indirection so that type parameters for a single
type-checked package can be stabilized.

This change should have no effect on non-generic type checking.

For #46003

Change-Id: I60d747e0a2bfb68e7d64e897eac23f609a2a4429
Reviewed-on: https://go-review.googlesource.com/c/go/+/321269
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api_test.go

@@ -353,7 +353,6 @@ func TestTypesInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		ResetId() // avoid renumbering of type parameter ids when adding tests
 		if strings.HasPrefix(test.src, genericPkg) && !typeparams.Enabled {
 			continue
 		}

src/go/types/api_typeparams.go

@@ -19,6 +19,11 @@ type (
 
 func NewSum(types []Type) Type { return _NewSum(types) }
 
+// NewTypeParam returns a new TypeParam.
+func NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
+	return (*Checker)(nil).newTypeParam(obj, index, bound)
+}
+
 func (s *Signature) TParams() []*TypeName           { return s._TParams() }
 func (s *Signature) SetTParams(tparams []*TypeName) { s._SetTParams(tparams) }
 

src/go/types/check.go

@@ -86,6 +86,7 @@ type Checker struct {
 	pkg  *Package
 	*Info
 	version version                    // accepted language version
+	nextID  uint64                     // unique Id for type parameters (first valid Id is 1)
 	objMap  map[Object]*declInfo       // maps package-level objects and (non-interface) methods to declaration info
 	impMap  map[importKey]*Package     // maps (import path, source directory) to (complete or fake) package
 	posMap  map[*Interface][]token.Pos // maps interface types to lists of embedded interface positions

src/go/types/type.go

@@ -732,11 +732,11 @@ func (t *Named) AddMethod(m *Func) {
 // Note: This is a uint32 rather than a uint64 because the
 // respective 64 bit atomic instructions are not available
 // on all platforms.
-var lastId uint32
+var lastID uint32
 
-// nextId returns a value increasing monotonically by 1 with
+// nextID returns a value increasing monotonically by 1 with
 // each call, starting with 1. It may be called concurrently.
-func nextId() uint64 { return uint64(atomic.AddUint32(&lastId, 1)) }
+func nextID() uint64 { return uint64(atomic.AddUint32(&lastID, 1)) }
 
 // A _TypeParam represents a type parameter type.
 type _TypeParam struct {
@@ -747,10 +747,17 @@ type _TypeParam struct {
 	bound Type      // *Named or *Interface; underlying type is always *Interface
 }
 
-// newTypeParam returns a new TypeParam.
 func (check *Checker) newTypeParam(obj *TypeName, index int, bound Type) *_TypeParam {
 	assert(bound != nil)
-	typ := &_TypeParam{check: check, id: nextId(), obj: obj, index: index, bound: bound}
+
+	// Always increment lastID, even if it is not used.
+	id := nextID()
+	if check != nil {
+		check.nextID++
+		id = check.nextID
+	}
+
+	typ := &_TypeParam{check: check, id: id, obj: obj, index: index, bound: bound}
 	if obj.typ == nil {
 		obj.typ = typ
 	}

src/go/types/types_test.go

@@ -4,14 +4,6 @@
 
 package types
 
-import "sync/atomic"
-
-// Upon calling ResetId, nextId starts with 1 again.
-// It may be called concurrently. This is only needed
-// for tests where we may want to have a consistent
-// numbering for each individual test case.
-func ResetId() { atomic.StoreUint32(&lastId, 0) }
-
 // SetGoVersion sets the unexported goVersion field on config, so that tests
 // which assert on behavior for older Go versions can set it.
 func SetGoVersion(config *Config, goVersion string) {